Guidewires are used in most catheter-based procedures. The distal end of a guidewire typically has an angled tip, which can be oriented to help steer the guidewire through curves and junctions of the vasculature or vessels of a patient. The orientation of the angled tip is achieved by torquing the guidewire so that it rotates about its axis.
Correct positioning of a catheter is dependent on the ability of the guidewire to track and be rotated to gain access to the target area. Guidewires generally will rotate in a 1:1 ratio between the proximal and distal ends in a straightened position, however, when subject to looping or bending as may occur in a tortuous anatomy, guidewires exhibit the tendency to whip (sudden release of torsional energy). This whipping makes precise access to target sites, such as selecting one vessel of a bifurcation, difficult.
Often, because of the variability of procedures and anatomy, the location of the hoop stress created by looping or bending of the guidewire cannot be predicted. Guidewires that are produced by maintaining a rigid proximal end and sequentially creating a more flexible distal portion are subject to whipping when looping or bending is applied away from the end of the distal portion.
One prior art device and method for torque transmission in a guidewire is shown in U.S. Pat. No. RE 36,628, to Sagae et al. The '628 patent teaches a catheter guidewire wherein the base material constituting the wire is made of an elastic alloy wire and is subjected to a heat treatment such that its flexibility is sequentially increased from its proximal to distal end portions. A thermoplastic resin and/or a coil spring may be applied to at least the distal end portion of the wire base material. A method of manufacturing the catheter guidewire is also taught. The method is characterized in that the leading end side of the base material is divided into a plurality of areas and subjected to a heat treatment by changing the heat treatment temperature and the time conditions in units of the areas so that the flexibility of the base material is sequentially increased from the proximal to distal end portions of the leading end side. As noted above, one of the drawbacks of guidewires such as that taught in the '628 patent is that guidewires that are produced by maintaining a rigid proximal end and sequentially creating a more flexible distal portion are subject to whipping when looping or bending is applied away from the end of the distal portion.
Another prior art device and method for torque transmission is taught in U.S. Pat. No. 5,951,494, which is hereby incorporated by reference. FIGS. 1-3 herein have been reproduced from the '494 patent. Referring to FIG. 1, a guidewire 2 includes a relatively stiff proximal portion 14, a transition portion 16 with varying, intermediate stiffness, and a highly flexible distal portion 18. The guidewire is formed entirely of common medical polymer materials and exhibits high torque fidelity because it has been twisted and tensioned in manufacture to helically orient the polymer. This is illustrated by a segment 8 of the wire that, prior to processing, was parallel to the device axis but after twisting and tensioning, follows a characteristic helical path.
Referring to FIG. 2, in the course of an angioplasty operation to open an occluded coronary artery, the guidewire 2 is typically delivered through an access catheter 20 into the femoral artery 22. The physician pushes and torques the proximal end of the guidewire to thread it through the body into the coronary arteries 24.
Referring to FIGS. 3A and 3B, the distal portion 18 of the guidewire is positioned such that it can cross a restricted region 28 of the artery. The physician pushes (arrow 30) and torques (arrow 32) the proximal portion of the guidewire remaining outside the body. The degree of rotation caused by torquing the proximal end is transmitted to produce a degree of rotation at the distal end.
As noted above, guidewires that are produced by maintaining a rigid proximal end and sequentially creating a more flexible distal portion are subject to whipping when looping or bending is applied away from the end of the distal portion. In addition, guidewires that are formed from certain elastic alloys are relatively expensive to produce. The present invention is directed to a device and method for overcoming the foregoing and other disadvantages. More specifically, the present invention is directed to a medical instrument such as a guidewire that is designed to have controlled torque transmission along its length.